“We were really acting out of an abundance of caution, because we don’t know enough about what these byproducts may do,” said Falmouth Board of Health chairman Jared Goldstone.

The byproducts, “small, chlorinated, organic molecules,” are a normal presence in Falmouth’s water, but levels rose above Environmental Protection Agency maximum levels in September 2013, Goldstone said in a phone interview Thursday. In anticipation of another rise in the byproducts this summer, the board decided to reissue the warning.

Get Fast Forward in your inbox:

Forget yesterday's news. Get what you need today in this early-morning email.

Pregnant women in their first and second trimesters should filter drinking water through a carbon filter, shorten the length of their showers, and ventilate showers well.

Aside from the potential risk to fetuses, high levels of exposure to these byproducts over a long period of time can increase the risk of some types of cancer, Goldstone said. The EPA regulations have been set to reduce the cancer risk, but because the studies of the effects on pregnant women are newer, the board is being cautious.

Goldstone said most people should not worry about the exposure, which must be long term to have negative effects.

A large portion of the town’s water comes from Long Pond, an open-surface source. To eliminate organic contaminants, Falmouth must chlorinate the water, Goldstone said, but the chlorine reacts with the organic matter to create the disinfection byproducts.

Falmouth is no longer in violation of the EPA limits but Goldstone said the levels are likely to rise this summer.

A water filtration plant, expected to cost the town more than $46 million, is on track to begin operation in Falmouth in 2017, eliminating the need for chlorine, Goldstone said.